1. Field of the Invention
The present invention relates to a seed layer structure for a spin valve sensor and, more particularly, to a trilayer seed layer structure which improves the magnetic and giant magnetoresistive properties and the thermal stability of the spin valve sensor by improving its microstructure.
2. Description of the Related Art
The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, a slider that has write and read heads, a suspension arm that supports the slider above the rotating disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The read head includes a sensor that is located between nonmagnetic electrically insulative first and second read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A write gap is formed between the first and second pole piece layers by a nonmagnetic gap layer at an air bearing surface (ABS) of the write head. The pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic field into the pole pieces that fringes across the gap between the pole pieces at the ABS. The fringe field writes information in tracks on moving media, such as in circular tracks on a rotating disk.
In recent read heads a spin valve sensor is employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer, respectively. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to an air bearing surface (ABS) of the head and the magnetization of the free layer is oriented parallel to the ABS but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling to an antiferromagnetic pinning layer.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layers are minimized. This thickness is less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layers are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos xcex8, where xcex8 is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor resistance changes cause potential changes that are detected and processed as playback signals by the processing circuitry.
The spin valve sensor is characterized by a giant magnetoresistance (GMR) coefficient that is substantially higher than the anisotropic magnetoresistance (AMR) coefficient of an AMR sensor. The GMR coefficient is xcex94RG/R∥ where xcex94RG is the difference between the resistance measured when the magnetizations of the pinned and free layers are antiparallel with respect to one another and the resistance (R∥) when the magnetizations of the pinned and free layers are parallel with respect to one another. A spin valve sensor is sometimes referred to as a GMR sensor. When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve sensor.
Another type of spin valve sensor is an antiparallel (AP) pinned spin valve sensor. The AP pinned spin valve sensor differs from the simple spin valve sensor in that an AP pinned structure has multiple thin film layers instead of a single pinned layer. The AP pinned structure has an AP coupling layer sandwiched between first and second ferromagnetic pinned layers. The first pinned layer has its magnetization oriented in a first direction by exchange coupling to the antiferromagnetic pinning layer. The second pinned layer is immediately adjacent to the spacer layer and is antiparallel exchange coupled to the first pinned layer across the AP coupling layer (having a thickness of the order of 8 xc3x85) between the first and second pinned layers. Accordingly, the magnetization of the second pinned layer is oriented in a second direction that is antiparallel to the first direction of the magnetization of the first pinned layer.
Antiferromagnetic nickel-manganese (Nixe2x80x94Mn), platinum manganese (Ptxe2x80x94Mn) and iridium manganese (Irxe2x80x94Mn) films have been used extensively as pinning layers for both simple and AP pinned spin valve sensors. The Nixe2x80x94Mn and Ptxe2x80x94Mn films must be annealed at about 280xc2x0 C. after deposition to cause a transformation from a nonmagnetic face-centered-cubic (fcc) phase to an antiferromagnetic face-centered-tetragonal (fct) phase. The anneal is not needed for the Irxe2x80x94Mn film which contain an antiferromagnetic face-centered-cubic phase after deposition. Spin valve sensors using an Nixe2x80x94Mn antiferromagnetic layer require post deposition anneals of about 12 hours at 280xc2x0 C. to develop a unidirectional anisotropy field (HUA) of 622 Oe, however this extended anneal causes a decrease in the GMR coefficient from 5.8% to 2.4% when a conventional tantalum seed layer is used.
Therefore, there is a need for an improved seed layer structure to allow Nixe2x80x94Mn spin valve sensors to be suitably annealed to develop a high unidirectional anisotropy field with good thermal stability without degradation of the GMR coefficient.
We found by employing a trilayer seed layer structure between a simple spin valve sensor having a nickel manganese (Nixe2x80x94Mn) pinning layer and an aluminum oxide (Al2O3) first read gap layer that the giant magnetoresistance coefficient (GMR) is 9.4% as compared to 5.8% and 6.8% when single seed layers of Ta and NiMnOx, respectively, are used. The trilayer seed layer structure includes a first seed layer made of polycrystalline nickel oxide (NiO), a second seed layer made of amorphous-like nickel manganese oxide (NiMnOx) and a third seed layer made of copper (Cu). The first seed layer interfaces the aluminum oxide (Al2O3) first read gap layer, the second seed layer interfaces the first seed layer, and the third seed layer is disposed between the second seed layer and the free layer. The trilayer seed layer may be employed in either a simple spin valve sensor or an antiparallel pinned spin valve sensor.
An object of the present invention is to improve the magnetic and GMR properties of a spin valve sensor when the pinning layer is made from a class of materials including nickel manganese (Nixe2x80x94Mn) and nickel manganese based alloys (Nixe2x80x94Mnxe2x80x94M) where M is a third metallic element such as chromium (Cr), iron (Fe), iridium (Ir), paladium (Pd), platinum (Pt), rhodium (Rh) and ruthenium (Ru).
Another object of the present invention is to provide a read head with a spin valve sensor that has improved thermal stability.
A further object of the present invention is to provide a seed layer structure for a spin valve having a nickel manganese (Nixe2x80x94Mn) pinning layer to improve its magnetic and GMR properties with good thermal stability.
Other objects and advantages of the invention will become apparent upon reading the following description taken together with the accompanying drawings.